For Option 1 of Navigation Tree (i.e by topic and then by type), each type that is under a topic needs to redirect to a Landing Page (see Proposal Landing Page and Continuation of Proposal Landing Pages). To do so, there are two options:
Option 1
Having 4 generic types (howto, explanation, reference, tutorials) that dynamically generate Landing Pages for the corresponding topic. For example: Howtos for users for the Page feature, Explanations for users for the Like feature.
@vmassol did a POC (see https://www.xwiki.org/xwiki/bin/view/XWikiOrg/DocumentationLandingPage/ ).
Pros:
- Avoids creating so many landing pages (there will be 4 pages per topic and there are hundreds of topics).
Cons:
- Readers lose where they are in the navigation tree.
- Not a nice breadcrumb (check an example).
Option 2
Creating dedicated Landing Pages directly under each topic.
Pros:
- Nice navigation and breadcrumb.
Cons:
- Too many landing pages.
- Readers can already filter from the LD on the topic Landing Page the type, for example check the LD on this Landing Page of a topic.
IMO the fact that readers can already filter by type in the LD of the topic Landing Page makes listing the type again in the navigation redundant, and to me this is generally a con of choosing the Option 1 Nav Tree, (but as you already know, I prefer Option 3 
).
WDYT? Thanks!